Pursuant to Jungian psychology, the collective unconscious is composed mostly of archetypes.

You can't learn about the shadow without knowing the archetype. It's like saying you want to learn about "opposite". Saying opposite or shadow doesn't make sense unless you first establish what you are referring to - the opposite of clean is dirty; the shadow of X archetype looks like ... .

The shadow doesn't happen in a vacuum, it's not random behavior that you get to label "shadow". It's psychology, these terms mean something, they have already been defined.

In the article I linked, the author explains all of this. The article also talks about the 4 universal archetype that we all share. One of which is the Saboteur.

You asked whether your friend eating candy is a form of sabotage. It could be the shadow aspect of the Saboteur archetype, or the shadow aspect of the Victim archetype, or the shadow of some other archetype.

You have to first figure out and learn about your own archetypes, and their shadow. Only then can you "see" how your archetypes relate to the archetypes of the people in your life.

Example: One archetype that people don't understand is the "Queen" archetype. People think that a person with the Queen archetype is getting waited on by others, that they never lift a finger, live a life of luxury, etc. That's not the Queen archetype, that's the Princess archetype. A Queen takes care of her people. It is a selfless archetype, not a pampered, selfish archetype. So, the Queen archetype is completely different from what people believe it to be.

A person with the Princess archetype will be unfulfilled, unless they grow into the Queen archetype. A Princess archetype that doesn't grow and remains stuck will likely live the shadow aspect of the Princess archetype.

Jungian psychology is a way by which you understand yourself and the world around you. You can't just talk about "shadow", or ask if some random behavior is the "shadow". That doesn't make sense because it simply doesn't work that way.